The present invention relates to a technique for measuring figure and placement errors of optical components constituting an optical system. The present invention further relates to an exposure method and an exposure apparatus for manufacturing semiconductor products by means of the technique.
At the present time, the reduction projection exposure method using ultraviolet light, which is widely used as a lithographic technique for fabricating semiconductor fine patterns, is said to have a resolution limit of the order of 0.1 .mu.m. A pattern with a resolution higher or better than 0.1 .mu.m cannot thus be created by a lithographic method using the ultraviolet light.
The reduction projection lithography using soft X rays with a wavelength in the range of 5 to 15 nm is, on the other hand, regarded as an effective technique for implementing the fabrication of such fine patterns. A typical configuration of a reduction projection exposure apparatus using such soft X rays is disclosed, for example, in Japanese Patent Laid-open No. 63-18626. Since an optical material with a refraction index usable in the soft X-ray region does not exist, however, a reflective optical system using curved reflectors is used in the X-ray reduction projection exposure apparatus for performing exposure of fine patterns in place of a refractive optical system utilizing lenses.
On the other hand, interferometric figure measurement is adopted in testification of the surface figure of a curved reflector employed in the optical system of the X-ray reduction projection exposure apparatus described above. A typical interferometric figure measuring apparatus is disclosed in Japanese Patent Laid-open No. 1-224306. In this interferometric figure measuring apparatus, a beam generated by a coherent light source is led to the reference surface of a half mirror. Part of the beam is reflected by the reference surface while the remaining portion of the beam is applied to a sample surface being measured where the beam is reflected thereby. These two reflected beams are superimposed on each other so as to result in interference. In this way, the surface figure of the sample can be measured by comparing it to the figure of the reference surface.
As for the method for evaluating an optical system, an article written in "Applied Optics, Vol. 13, No. 11, 2693-2703 (November 1974)" is a typical source of information that can be used as a reference.
In the X-ray reduction projection exposure apparatus disclosed in Japanese Patent Laid-open No. 63-18626, stress deformation of the reflector occurs due to a fixed force and its weight at the setting-up time of the reflector. In addition, thermal deformation also occurs during the exposure process as well. It is thus necessary to have an extremely high reflective-surface accuracy as well as an extremely high setting-up accuracy of the reflector in the X-ray reduction projection exposure apparatus. However, the X-ray reduction projection exposure apparatus has a problem in that it is difficult to sustain these kinds of accuracy in the course of exposure.
In addition, when evaluating the reflective-surface figure of the reflector employed in the X-ray reduction optical system, it is necessary to avoid deformation effects of the reflector in the setting-up thereof. It is therefore necessary to carry out the evaluation with the reflector set up on the apparatus as it is. In the case of the conventional interferometric surface measurement technique, however, it is necessary to install measuring optics comprising, among other components, a reference surface so as to expose the measuring optics to a surface being measured. As a result, the conventional interferometric technique has a problem of mechanical interference with another reflector employed in the X-ray projection optical system. It also has a problem in that no optical path can be secured for the measurement.
Further, wave-front distortion measurement adopting the conventional interference method can be applied to an optical system comprising a plurality of optical surfaces to provide wave-front distortion data which includes information indicating whether or not the optical characteristics of the optical system as a whole are good. However, wave-front distortion does result from superimposition of all effects of figure and placement errors. Accordingly, the wave-front distortion measurement also has a difficult problem that it provides no information as to how much the figure and position of each optical surface should be corrected.